The biogenesis of mitochondria and chloroplasts is genetically complex, involving hundreds of genes distributed between the nucleus and organelle. In multicellular organisms, developmental parameters confer an added layer of complexity upon the genetic control of organelle biogenesis; the properties of plastids, in particular, differ dramatically between different cell types. The biochemistry and structure of different plastid types have been described in detail. Mutant phenotypes can now direct us to otherwise hidden functions that determine the timing and localization of chloroplast development, and that mediate their assembly. The sophisticated genetics of maize, its well characterized transposons, large seed, and mode of leaf development, together make it uniquely suited for the study of developmental aspects of organelle biogenesis. Twenty-eight nuclear mutants have been isolated from a line of maize harboring the transposon Mu, that lack various subsets of chloroplast proteins. This proposal focuses on six of these mutants whose phenotypes suggest lesions in a post-translational process involved in thylakoid membrane assembly, such as protein targeting or assembly of multisubunit complexes. Their genetic properties will be further studied to identify those that behave convincingly like Mu-induced lesions. Detailed analyses of the mutant phenotypes at the molecular, biochemical, and structural levels will provide clues concerning the immediate function of genes defined by these mutations. Genetic crosses will be performed to map the mutations to a chromosome arm, to identify independent alleles, and to assess the effect of gene dosage on phenotype. Developmental studies will address the timing of gene action during leaf development and whether the gene acts specifically on chloroplasts, or on non-green plastids as well. Molecular clones of several of the most interesting Mu-induced mutations will be isolated by taking advantage of the transposon tag. These clones will be used to obtain the corresponding normal alleles, which will then be used as tools for more detailed functional studies.